1. Field of the Invention
The present invention relates to an output circuit which can suppress ringing of its output voltage waveform, and more particularly, it relates to a technique of suppressing vibration such as ringing, overshooting or undershooting which is caused in the voltage waveform by transient response resulting from conductance, inductance etc. of load capacitance, wires etc. connected to an output terminal, impedance mismatching between the output circuit and the load, and the like.
2. Description of the Background Art
In recent years, a high-speed operation is required for a logic circuit due to a demand for high-speed processing of digital data. Also in an output circuit, therefore, it is necessary to drive a load having high capacitance at a high speed.
When an output circuit which is connected with high load capacitance is driven at a high speed, however, the output circuit inevitably energizes a resonance circuit which is formed on the load side by inductance of the load capacitance, wires and the like, to cause ringing in the output waveform thereof.
In order to increase the operating speed of such an output circuit, further, it is necessary to regard the load connected to the output circuit, which can be handled as a lumped parameter element having simple inductance and capacitance, also as a distributed parameter element. For example, wires provided on a multilayer substrate must be handled as microstriplines, which are distributed parameter elements.
FIG. 54 shows an exemplary CMOS output circuit of a conventional MOS integrated circuit. Referring to FIG. 54, the CMOS output circuit comprises an N-channel transistor 100N which outputs a low level and a P-channel transistor 100P which outputs a high level. The transistors 100P and 100N have gates which are connected to an input terminal 11, drains which are connected to an output terminal 12, and sources which are connected to a ground potential 2 and a power source 3 respectively. These transistors 100P and 100N are complementarily driven by an input signal supplied to the input terminal 11, which is connected to the gates of these transistors 100P and 100N in common. When the input terminal 11 receives a high-level input signal, the transistor 100N is driven and the other transistor 100P not driven so that the logical level of the output terminal 12 goes low, and vice versa.
In order to drive a load by the conventional output circuit at a high speed, the N-channel transistor 100N and the P-channel transistor 100P may be increased in current drivability by increasing ratios of channel widths channel lengths, for example. In general, the channel widths of such transistors are increased. When such transistors are simply increased in current drivability, however, ringing may disadvantageously be caused in the output waveform, depending on a load which is connected to the output terminal 12.
FIG. 55 illustrates an equivalent circuit of the output circuit shown in FIG. 54, which is connected with a load to be driven, with a simple model. This equivalent circuit is in a state outputting a low level. The N-channel. transistor 100N is expressed by parallel connection of a current source CSU and ON resistance R.sub.ON. A load capacitor 320 having a conductance value C is connected to the output terminal 12 through an inductor 310 having an inductance value L.
The inductor 310 is a parasitic inductor which is formed by a wire, a copper foil wire provided on a printed board, a bonding wire for an integrated circuit, or the like.
As clearly understood from FIG. 55, the output circuit including the load forms a resonance circuit.
A resonance frequency f.sub.0 of the equivalent circuit of this model is expressed as follows: ##EQU1## (f.sub.0 :the resonance frequency)
Assuming that 2.pi.f.sub.0 =.omega..sub.0, the Q value in the resonance frequency f.sub.0 is expressed as follows: ##EQU2##
The ON resistance R.sub.ON of the output transistor is reduced as the current drivability of the transistor is increased for increasing the speed of the output circuit. FIG. 56 shows output voltage/output current characteristics of the output circuit which is increased in current drivability. Referring to FIG. 56, a line 201 represents the characteristics provided before the current drivability is increased, and symbol .theta..sub.1 represents an inclination showing the inverse number of an ON resistance value of the output circuit in the vicinity of a low level measured before increase of the current drivability. On the other hand, a line 202 represents the characteristics provided when the current drivability is increased, and symbol .theta..sub.2 represents an inclination showing the inverse number of an ON resistance value of the output circuit in the vicinity of a low level measured after increase of the current drivability. As understood from FIG. 56, the ON resistance (output voltage/output current) is reduced and that in the vicinity of a low level is also reduced when the current drivability is increased. When the ON resistance is thus reduced, the Q value is increased as understood from the above expression 2, whereby the resonance circuit is energized by abrupt change of the output signal, which tends to vibrate. In other words, ringing, overshooting or undershooting is caused in the output waveform.
On the other hand, ringing may also be caused in the output waveform by impedance mismatching when the load, such as a transmission line, must be handled as a distributed parameter element. In this case, such ringing causes a noise in the transmission line to result in a malfunction of the logic circuit system, or an interference wave, called undesired radiation, to other electric apparatus.
FIG. 57 shows exemplary ringing resulting from simulation of an output waveform.
In the conventional CMOS output circuit having the aforementioned structure, ringing or the like is caused in its output waveform when the current drivability of the output transistor is increased in order to attain a high speed.